Early defibrillators provided only monophasic waveforms. USRE27652 to Mirowski (priority 09 Feb 70) refers to an automatic defibrillator with a monophasic shock circuit which delivered an untruncated shock as soon as the storage capacitor charged to a fixed voltage (no isolated control signal was needed). FR2257312 to Zacouto (priority 16 Jan 74) refers to providing sequential monophasic shocks over multiple electrode pairs, also not isolating control. U.S. Pat. No. 4,403,614 to Engle (priority 19 Jul. 79) and U.S. Pat. No. 4,384,585 to Zipes (priority 06 Mar 81) referred to synchronizing shock with detected events, but did not show any details of the discharge circuit. U.S. Pat. No. 4,614,192 to Imran (priority 21 Apr. 82) refers to truncating monophasic shocks by rapidly discharging the storage capacitor. The shock switch and driver consisted of a pulse transformer driving a silicon controlled rectifier (SCR), a pulse transformer controlling a thyristor.
Following experiments with bidirectional shocks in 1964 and 1980, J. C. Schuder et al. described an "Ultrahigh-energy thyratron/SCR bidirectional waveform defibrillator", in Med Biol Eng Comput 20:419, 1982, having a biphasic generator with one capacitor per phase. SU1149979 to Pekarski (priority 08 October 83) also refers to a biphasic truncated shock circuit with one capacitor for each phase.
In 1984, Schuder et al. presented results of a simulated single-capacitor truncated biphasic waveform. In their paper entitled "Transthoracic Defibrillation of 100 Kg Calves with Bidirectional Truncated Exponential Shocks", Vol XXX Trans Am Soc Artif Intern Organs, 1984, the authors referred to experiments made with an "asymmetrical truncated exponential biphasic waveform . . . which can be implemented in a clinical sized apparatus." They showed a waveform where the trailing edge of the first phase was equal to the leading edge of the second phase.
The single capacitor approach simplifies both charging and discharging circuits, reducing size, weight, and unreliability in implantable devices. As data accumulated showing improved animal and clinical results with biphasic truncated shocks, compared to monophasic truncated shocks, there have been proposed a variety of single-capacitor multiphasic truncated waveform generators. All such circuits include at least four switches in an H-bridge configuration (also referred to herein as an "H-bridge switch").
Designers frequently employ the H-bridge configuration for driving a load in two directions from a DC source, for example, driving a stepper or servo motor from a battery. In the first phase a first switch connects the positive source pole to a first side of the load and a second switch connects the negative source pole to the second side of the load. In the second phase a third switch connects the positive source pole to the second side of the load, and a fourth switch connects the negative source pole to the first side of the load. The first and third switches, connected to the positive source pole, are called high side switches. The second and fourth switches, connected to the negative source pole, are called low side switches.
Prior art implantable discharge circuits employ one or more of three types of switches in the H-bridge. Each type of switch has an input, output, and control terminal, and responds to a control signal between the control and output terminals. Silicon controlled rectifiers (SCRs) turn on in response to a pulse on the control terminal, but only turn off when current through them falls essentially to zero. Metal-oxide-semiconductor field effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs) remain on while a control voltage appears at the control terminal.
Depending on how they protect pacing and sensing circuits from defibrillation pulses, prior art circuits either isolate the capacitor and discharge circuit from pacing and sensing ground, or they connect the negative side of the capacitor to ground. In the isolated version they must provide isolated switch control signals. In the negative-ground version, they must still provide isolated control signals to the high side switches.
Thus, any single capacitor biphasic shock delivery circuit needs: two high side switches and two low side switches connected in an H-bridge, and at least two isolated switch drivers. The following prior art patents all disclosed an H-bridge for generating a single-capacitor multiphasic waveform, where the structure for the H-bridge switches and the switch control drivers differ in each design.
U.S. Pat. No. 4,800,883 to Winstrom (priority 02 April 86) refers to an isolated discharge circuit with four MOSFET switches, and a transformer with an RF carrier, rectification, and rapid shutoff circuits for high and low side drivers. A single transformer with two secondaries drives both high and low side switches in the same phase. A multilevel capacitor with voltage taps is described.
EP0281219 to Mehra (priority 14 Jan 87) refers to a negative-ground discharge circuit with an SCR in series with a MOSFET for each high side switch and an SCR for each low side switch. Mehra did not give details of the switch drivers.
EP0280526 to Baker (priority 27 Feb 87) refers to using the Winstrom circuit above, with the additional requirement of a first phase duration that is longer than the second phase duration (note that in 1984 Jones et al. published results for defibrillation pulses with a 5 ms first phase and a 1 ms second phase, see Am. J. Physiol. 247 (Heart Circ. Physiol. 16)). Baker also refers to providing protection against a short-circuited load, by opening the H-bridge switches when the load current exceeds a preset value.
EP0324380 to Bach (priority 12 Jan 88) provided another negative-ground discharge circuit, with SCRs for high side switches and MOSFETs for low side switches. Bach used pulse transformers for high side drivers and drove the low side directly. Bach included diodes in series with low side switches to protect against external defibrillators.
EP0326290 to de Coriolis (priority 19 Jan 88) provided yet another negative-ground discharge circuit, with two SCRs in series for the first phase high side switch, a MOSFET for the first phase low side switch, and SCRs for the second phase high and low side switches. de Coriolis truncated the second phase by rapidly discharging the storage capacitor through the first phase high side switch and the second phase low side switch. de Coriolis drove the high side switches with pulse transformers and the low side switches with level shifters referred to a positive supply.
U.S. Pat. No. 4,998,531 to Bocchi (priority 28 Mar 90) provided still another negative-ground discharge circuit, with four MOSFET switches. Each MOSFET switch had a series diode to prevent reverse current during external defibrillation. Bocchi used level shifters for low side drivers and used a transformer for the high side driver, where a pulse in one direction turned the MOSFET on, and a pulse in the other direction turned the MOSFET off.
U.S. Pat. No. 5,111,816 to Pless (priority 22 October 90) provided yet another negative-ground discharge circuit, with IGBT or MOSFET switches. All Pless variants drive both high and low side switches in the same phase from a common transformer with an RF carrier and rectification, and a rapid shutoff circuit for at least one switch in each phase. Pless also referred the negative battery terminal to ground and inverted this to make the pacing voltage.
All prior art designs either isolate the discharge circuit from pacing and sensing ground, or refer the negative pole of the storage capacitor to pacing and sensing ground. This requires electrically isolating control signals for high side switches. A problem with the prior art designs is that they use transformer coupling for isolation. They use pulse transformers to drive SCRs, and either pulse transformers or RF transformers with rectification and a rapid shutoff circuit to drive MOSFETs or IGBTs.
The disadvantages of such transformer coupling include magnetic coupling from other inductors or transformers in the implant, such as the transformer which charges the energy storage capacitors; magnetic coupling to sensitive circuits elsewhere in the implant, such as current loops in the high-gain R-wave sensing circuits; relatively bulky and expensive magnetic components which cannot be implemented using integrated circuit technology; the possibility of transformer core saturation in a strong external DC magnetic field, including fields produced by permanent magnets commonly used to test pacemakers and defibrillators; and magnetic coupling from strong external AC magnetic fields, such as fields produced by industrial heating or welding apparatus.
In prior art designs the transformer also required complex and power-hungry additional radio frequency oscillator or pulse driver circuits.
There is thus a continuing need for improvement of high-voltage shock circuits for use in implantable defibrillators.